Roman's answer looks ideal. There is another less elegant way. I'm not sure where icons live in the rest of the system, but icons that are on your Desktop are just little plain text files in $HOME/Desktop. They usually (but don't have to) have the extension of .desktop. One of the fields in these files is Icon=.... You can use a program to change whatever that field is set to to the value of your choice. (You may have to do a refresh after doing this just as Roman suggests.)

When I want to use an icon for something else or on another system, I sometimes look in these .desktop files to find out what the icon in use is named.